Fix three tester findings on T4 activation gate:
1. Real serving enforcement: evaluateActivationGate now calls
gateServing() → DisconnectVolume(iqn) on gate (terminates active
iSCSI sessions, removes volume from target). ungateServing() →
AddVolume(iqn, adapter) on clear (re-registers volume). This is
actual serving enforcement, not just bookkeeping.
2. Wire propagation: add activation_gated (field 25) and
activation_gate_reason (field 26) to proto BlockVolumeInfoMessage.
Add generated Go fields + getters. Add proto conversion in
InfoMessageToProto/InfoMessageFromProto. Gate state now rides the
real VS→master heartbeat wire.
3. Runtime ungate: evaluateActivationGate() now also runs in
applyCoreEvent() (the observation-driven path), not just
applyCoreAssignmentEvent(). Recovery/catch-up completion that
transitions the projection to publish_healthy/replica_ready now
clears the gate and re-registers the volume automatically.
ClearActivationGate() remains as an explicit override for edge cases
but is no longer the primary ungate path.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
